7. What are the characteristics of personal music players

Personal music players
have a very wide field of application ranging from professional
tools at the workplace to the leisurely consumer and to children
who use these devices as toys. They are portable digital music
players that play music as audio files, such as
MP3. In addition, most of
these devices allow to store video, pictures, and to receive
radio and TV programs (podcasting). Earphones and external
speakers are the typical output devices delivering
sound to the listener.
Personal music players (PMPs) are widely used in conjunction
with several headphones of different styles (insert,
supra-aural, vertical, and circumaural).

To identify the risk levels of PMPs one has to realize that
the chain of music reproduction is organised in stages which are
more or less independent of each other but together affect the
output signal level.
Personal music players
reproduce music from a recording. The
sound of a signal has
usually been recorded through a microphone and the oscillations
resulting from the pressure changes are stored as a sampled
wave form. Different
procedures and algorithms are used for storing of analogue or
digital representations of the wave forms which are offering low
or high data compression and provide the means for
reconstruction of the original waveform during play-back. The
sound level is not
significantly affected by the compression algorithm (if any) and
by the degree of compression used in a particular recording.
Often recordings are made by mixing samples from a number of
microphones.

The dynamic range of a recording is generally chosen to fit
the technical characteristics of the storage device (e.g. music
cassette tape) and depending on the tone engineer and the style
of the music. Thus, the full dynamic range of a ‘life music’
situation is typically compressed to a variable degree into the
dynamic range available for recording and play back.

Using the appropriate algorithm, the recorded signal is
re-synthesized into a waveform, typically, in the form of an
analogue electronic signal, by the player. Manufacturers of
players usually specify the electronic characteristics of the
output (e.g. maximum voltage, impedance) of their
equipment.

To produce sound the
player is connected to a transducer: headphones, earphones or
earbuds. Important for our purposes is how effective the
transducer is in transforming voltage into sound energy.

As said above, two matters are important. The electronic
coupling of the transducer to the player may affect the output
of the player. This is technically a matter of input and output
impedances and can be described as a coupling factor. The
transducer produces a certain amount of
sound energy for a given
electronic energy (voltage times current drain) delivered to the
input. This is the sensitivity of the transducer.

The earphones or earbuds are inserted in the
ear. The place in the
ear canal also determines
the effectiveness of the transfer from transducer to the
tympanic membrane. It seems evident that a circum-aural
headphone has to produce more energy than an earbud inserted in
the ear canal, simply because of the larger volume of air that
has to be excited.

The volume of the sound
emitted by PMPs varies from manufacturer to manufacturer, and is
difficult to estimate.

With the currently available digital formats (e.g.
MP3) of
sound recording and
reproduction, it is possible to reach high levels of sound
output without distortion. The
personal music players now
play not only music, but provide podcasts of various broadcasts
or lecture material, which is delivered largely through
ear-bud type insert ear
phones producing a range of maximum levels around 80-115
dB(A) across different
devices. Sound pressure levels change with the insertion depth
of the ear-bud in the
ear canal, the maximum
output provided by the particular device and ear-bud combination
and the type of music.

Fligor and Cox (2004) tested some devices by different
manufacturers and style of headphones. They found that
free-field equivalent sound pressure levels measured at maximum
volume control setting ranged from 91
dB(A) to 121 dB(A).
Moreover, they estimated an influence of 7-9
dB with an
ear-bud type producing the
highest levels in the
ear canal.

3.6.3. Conclusions

With the currently available digital formats (e.g.
MP3) of
sound recording and
reproduction, it is possible to reach high levels of sound
output without distortion (around 80-115
dB(A) of maximum levels
across different devices) and the difference in
ear-phone type may
increase that level. These levels change with the insertion
depth of the ear-bud in the
ear canal, the volume
setting of the device, the maximum output provided by the
particular device and ear-bud combination and lastly the type of
music. In the worst case scenario, it is possible to estimate
maximum levels of about 120 dB(A).